It is often desired to drive piles into the ocean floor to depths of several hundred feet or more to anchor a tower or platform structure from which oil and gas wells can be drilled and operated. An exemplary oil or gas well tower of a type used in water of medium depth, typically about 400 to 500 feet, is described in U.S. Ser. No. 3,895,471. It is anchored by a circular array of piles, one at each of four corners. The tower is wider at its base than at the top and each pile is driven at an angle to the vertical that is approximately aligned with an imaginary line connecting outer edges of the tower at the top and at the bottom. Although it is more difficult to drive the piles at such an angle, it is thought that greater holding power results. It should be noted that the piles do not extend to the top of the tower but terminate at the top of relatively short pile-receiving guides that form part of the base structure of the tower.
A tower of this general construction is said to be "nailed" to the ocean floor and is not to be confused with the guyed tower type of construction in which the piles extend to a point above the ocean floor and form an integral part of the entire tower structure. As a rule, guyed towers are suitable for use in greater water depths, such as one thousand feet.
The present state of the art calls for pounding the piles by the repeated blows of a hammer. Each blow may contain more than one million foot pounds of energy, but at deep penetration drives the member only a fraction of a foot.
Conventionally, a hammer and its leads, which may weigh 400 tons or more, must be supported above a pile by a crane mounted on a barge. The further into the ocean floor a pile is driven, the greater the force required to drive it and the larger the hammer must be. Some experts believe that a large portion of the hammer energy is absorbed by radial movement and vibration of the pile throughout is length. In the case of a "nailed" tower, additional energy is absorbed by a long follower that transmits the force from the above-water hammer to the top of the pile. The use of a hammer is greatly complicated by the movement of the barge and crane relative to the pile due to wind and water currents. Limited use has been made of underwater hammers.
Many areas in which towers are located frequently experience severe storms. It is, therefore, necessary to wait for a suitable "weather window" during which to erect the tower and drive the piles. As the time required to drive the piles increases, the necessary window becomes larger. The difficulty of finding such a window increases as does the chance of an unexpected storm that could prove disasterous. It is, therefore, important to drive the piles as rapidly as possible so that the structure can withstand heavy seas if necessary. It is also highly desirable to have an effective technique for anchoring the tower to any partially driven piles in the event of an unexpected storm.
There are important disadvantages associated with conventional hammer-driven piles that relate to their essential purpose of securing the tower. When the pile is hammered, it unavoidably moves radially as it abruptly surges downwardly with each blow. In so doing, it disturbs the soil around it, and may leave an annular space between the pile and the soil which reduces soil friction. Although the soil may regain part of this initial strength as it settles, some loss is permanent. The result is that the forces and energy required to remove the pile are less than that required to drive it and the holding power of the pile is not accurately predictable, even if the energy used in driving it is known.
Another problem experienced with hammer-driven piles is that the numerous variables make it difficult or impossible to accurately monitor the force required to drive the pile at successive penetration levels. For this reason, existing techniques that attempt to predict the static-bearing capacity of a pile based on the history of its dynamic drive resistance are not totally reliable. To compensate for this unreliability, large safety factors must be included in design specifications. In some situations, a pile is driven at considerable cost to a predetermined depth far greater than that required to secure the tower when soil conditions offer more resistance than expected.
Objectives of the present invention are to provide new methods and apparatus for driving piles that secure oil and gas well towers and similar structures. A further objective is to utilize apparatus that is of less weight, has lower energy requirements, and is more easily managed. Other objectives are to drive the pile in a manner that minimizes the disturbance of the soil surrounding it and renders the holding power of the pile more predictable.